-- Last Updated: May-04-10 12:09 PM EST --
...depending on the size of your lake and the class of your river. Assuming your river doesn't go above class 1 and you're not expecting to cross large lakes, something in the 14' to 16' range with shallow arch or shallow vee bottom and mild rocker. You might want to avoid flat bottoms. They have their place, but are not versatile enough for what you want. If they are flat with no keel they will hard to manage on windy lakes, and if they're flat with a keel they will be hard to handle on twisty rivers.
Don't put too much stock in "wide as possible". 36" is plenty wide. The 14' Wenonah Fisherman is 39" wide and I would avoid going any wider on any canoe.
Any tandem canoe can be soloed, but few solo canoes will work as a tandem. There are several methods for soloing in a tandem, an which works best depends on you and which boat you get - point being that you don't need to be concerned about that when deciding.
15' to 16' light tripping canoes are great for fishing and easy river running - tandem or solo. Lots of boats by Old Town, Wenonah, Mad River, Bell, Nova Craft, and others that can be found used in this category. Unless you don't mind heavy poly hulls, used is a better way to save money. Even so, a used poly hull should be cheaper still, and should work just fine unless it was terribly abused.
You will find there is no perfect boat unless you limit yourself to one kind of water. Shopping for used also might allow you to have more than one, with each leaning towards opposite ends of the spectrum of your expected activities.
For a good example of a competent all-rounder that you prbably won't get bored with any time soon, look at Old Town's 16' Penobscot (a pretty common boat). Some folks will tell you that the roundish bottom on the Penobscot makes it too "tippy" for fishing, but I disagree. I've used the flatter and wider Wenonah Fisherman also, and would say that either is as good as the other for fishing, although the Fisherman might be better for photography and birdwatching on *calm* water.
The Penobscot is faster and tracks better, while the Fisherman (though slow by comparison) is more suitable for tight spaces and extremely shallow water. I can stand and fish in either, solo of course (expect you can too with a little practice, if you are reasonably fit), - which should tell you something about the latitude you have to work with.